Process for removing ammonia from waste waters

ABSTRACT

A method for removing ammonia, or ammonia and phosphates from industrial agricultural or household effluents by causing precipitation as magnesium ammonium phosphate by means of a liquid reagent containing magnesium and phosphate ions and an acid. The effluents have more than 1 gle of ammoniacal nitrogen and the pH is adjusted to between 9 and 11 after addition of the liquid reagent.

The invention relates to a process for removing ammonia from wastewaters.

The removal of ammonia from waste waters is generally carried out by abiological treatment. Waters heavily laden with ammonia, containing morethan one gram of ammonia per litre, are lethal to living organisms. Inorder to carry out the biological treatment of these waters, it isnecessary to modify the process and to add nitrification anddenitrification stages, which leads to high additional costs.

There are also physical processes for removing ammonia.

The stripping of ammonia with air or with steam requires majorinvestment, which can only be borne by large plants.

Ion exchange is only conceivable for low concentrations. In fact,because of difficulties in treating the regeneration eluates, theproblem is simply shifted.

Membrane techniques, such as electrodialysis, give goods results butstill leave many questions unresolved, concerning the service life ofthe membranes, blockages, etc. The cost of these techniques is alsohigh.

German Patent Application DE-A-3,732,896 describes a chemical processbased on the formation, by successive addition of a phosphate and thenof magnesia to waste waters, of a salt, magnesium ammonium phosphate,which is slightly soluble in aqueous media. This process does not allowthe complete removal of the ammonia contained in the waste waters. Withthe aim of improving this process, and thus of increasing the degree ofremoval of the ammonia, European Patent Application EP-A-335,280, by thesame inventor, proposes a three-stage process. This process consists ininserting an acid treatment between the phosphate addition and magnesiaaddition stages. This three-stage process is more complex and thuscostly, and requires the handling of two solid reactants.

We have now found a process for removing ammonia in a single stage,using a single reactant.

One great advantage of this process is the use of a liquid reactantwhich is easy to handle and to measure.

Another advantage of this process is a degree of removal of ammoniawhich is markedly superior to that of the prior art.

An advantageous possibility is to follow the process according to theinvention with a biological treatment, made possible by the reduction inthe ammonia content of the treated waste waters.

Finally, it is very important to note that the process allows ammoniaand phosphates to be removed from waste waters simultaneously.

The process according to the invention for removing ammoniacal nitrogencontained in waste waters consists in the precipitation of theammoniacal nitrogen in the form of a magnesium ammonium phosphate,characterised by the addition to the waste waters of a single liquidreactant containing sources of magnesium ions, phosphate ions and anacid.

The magnesium ammonium phosphate corresponds to the formula NH₄ MgPO₄.It generally crystallises in the hexahydrated form. This salt is verysparingly soluble in water and it is even used in analytical chemistryfor the determination of magnesium or of phosphoric acid.

The single liquid reactant preferably contains equimolar quantities ofphosphate and magnesium ions. This corresponds to the stoichiometry ofmagnesium ammonium phosphate. The phosphate/magnesium ratio can howevervary between approximately 0.5 and 1.5.

If the waste waters also contain phosphate ions in addition to ammonia,this is taken into account in the formulation of the reactant in orderto obtain the desired stoichiometry.

The reactant generally contains approximately 0.8 to 2 moles ofmagnesium ions and of phosphates per mole of ammonium ion. An excess ofmagnesium and of phosphate ion in relation to the ammonium ions isgenerally used.

In general, an excess of 1.1 to 1.5 moles of magnesium ions and ofphosphates per mole of ammonium ions is sufficient.

The source of magnesium ions is generally magnesium oxide or magnesia,MgO. It is, however, possible to use magnesium salts, such as thechloride or the sulphate.

The source of phosphate ions is generally phosphoric acid, H₃ PO₄, butalkali metal or alkaline-earth metal phosphates are equally suitable.

The acids are strong inorganic or organic acids, such as hydrochloricacid, sulphuric acid, formic acid or acetic acid. Hydrochloric acid ispreferably used. The quantity of acid necessary to obtain a pH ofbetween approximately 1 and 2 is used.

The single liquid reactant is prepared by simply mixing the sources ofmagnesium ions and of phosphate, the strong acid and, optionally, water.

The ammoniacal nitrogen content of the waste waters is determined bychemical or physical measuring and the reactant containing between 0.8and 2 moles, and preferably between 1.1 and 1.5 moles, of magnesium ionsand of phosphates in relation to the ammoniacal nitrogen content is thenintroduced.

With an excess of reactants of approximately 50%, the magnesium ammoniumphosphate precipitates virtually quantitatively. By adding the sameexcess of magnesium ions and of phosphates, but separately, in twostages, only 90 % of the nitrogen contained in the waters is removed.

After addition of the single reactant, the pH of the medium is adjustedwith a base. The pH must be greater than 7, preferably greater than 9.It is advantageously between 9 and 11.

In order to adjust the pH, an inorganic base, such as sodium hydroxide,potassium hydroxide or mixtures thereof, is used. It is also possible touse bases recovered from industrial operations, such as waterscontaining soda, without prior purification.

Since the salt formation is exothermic, it is pointless to heat themedium. The salt precipitates instantaneously. It is separated from thewaste waters by a physical process, such as filtration, centrifuging orsettling.

The process can operate either continuously or noncontinuously. In thecase of continuous operation, the nitrogen analyser, which continuouslymeasures the nitrogen content of the waste waters, directly actuates aninlet microvalve for the single reactant. The pH is adjusted in thereactor by addition of a base and the salt is then separated, preferablyby settling.

The process according to the invention allows the treatment of all wastewaters containing nitrogen in the form of ammonium ions. It isparticularly suited to the treatment of waste waters containing morethan 1 g/1 of nitrogen in the ammoniacal form and to the treatment ofwaters containing ammonium ions and phosphate ions.

The waste waters may be of industrial, agricultural or domestic origin.

Waste waters heavily laden with ammonia, of industrial origin, originatefrom industries which use animal or plant materials as well as fromchemical industries which handle nitrogenous products. These include theprimary agro-foodstuffs industries such as pectin or gelatin factoriesor slaughterhouses. Among the chemical industries, fertilisermanufacture may be mentioned.

Agricultural waste waters arise from the animal rearing sector, pigslurry manure liquids being the best known example.

The following examples illustrate the invention without, however,limiting it.

EXAMPLE 1

A solution of ammonium chloride in water is prepared, which contains 1g/l of nitrogen in the form of ammonium ions.

The reactant is prepared from 30 g of water, 8.2 g of phosphoric acid, 3g of magnesia and 8.2 g of hydrochloric acid. 50 g of a reactant is thusobtained which contains 75 mmol of magnesium and 75 mmol of phosphate.

This reactant is added to the ammonium chloride solution and the pH isadjusted to 10 with sodium hydroxide solution.

Table 1 summarises the purification yields as a function of the excessof the reactant employed.

EXAMPLE 2 (Comparative)

A solution of ammonium chloride in water is prepared, which contains 1g/l of nitrogen and is identical to Example 1. Variable quantities ofdipotassium phosphate are added, followed by equimolecular quantities ofmagnesium sulphate. The pH is adjusted to 9.9 with sodium hydroxidesolution.

Table 1 summarises the purification yields as a function of the excessof the reactant employed.

EXAMPLE 3

To an aqueous ammonium chloride solution, identical to Examples 1 and 2,is added a single reactant, containing 40 g of water, 18 g ofdipotassium phosphate, 13 g of magnesium sulphate and 4 g ofhydrochloric acid. A reactant containing 75 mmol of magnesium and 75mmol of phosphate is thus obtained. This reactant is added to theammoniacal solution and the pH is adjusted to 9.9 with sodium hydroxidesolution.

Table 1 summarises the purification yields as a function of the excessof the reactant employed.

                  TABLE 1                                                         ______________________________________                                                  Purification                                                                            Purification   Purification                                         yield %   yield % Example 2                                                                            yield %                                    Molar excess                                                                            Example 1 (Comparative)  Example 3                                  ______________________________________                                         0%       76        58             80                                         10%       85        66             82                                         25%       92        76             88                                         50%       >99       92             92                                         ______________________________________                                    

A reactant containing 75 mmol of magnesium and 75 mmol of phosphate isthus obtained.

EXAMPLE 4

The procedure of Comparative Example 2 is used with a water sample,originating from the combustion condensates of a meat cooker, whichcontains 1.8 g/l of nitrogen in ammoniacal form.

With an 80% excess of dipotassium phosphate and of magnesium sulphate,the yield of the purification is 91%.

EXAMPLE 5

The procedure of Example 1 is used on a water sample arising fromaerocondensers, which contains 556 ppm of nitrogen in the ammoniacalform per litre.

With a 30% excess of reactant, the purification yield is 96%.

We claim:
 1. A process for removing ammoniacal nitrogen contained inwaste waters having more than 1 g/l of nitrogen in the ammoniacal formby precipitation in the form of a magnesium ammonium phosphate,comprising the addition to the waste waters of a single liquid reactantcontaining sources of magnesium ions, phosphate ions and an acid, andthen adjusting the pH to a value of between 9 and
 11. 2. The processaccording to claim 1, wherein the ratio of the phosphate/magnesium ionsof the reactant is from 0.5 to 1.5.
 3. The process according to claim 2,wherein the ratio of the phosphate/magnesium ions is 1/1.
 4. The processaccording to claim 3, wherein the reactant contains between 0.8 and 2moles of magnesium and of phosphate per mole of ammoniacal nitrogen inthe waste waters.
 5. The process according to claim 1, wherein thesource of magnesium ions is selected from the group consisting ofmagnesium oxide, magnesium sulphate, or magnesium chloride.
 6. Theprocess according to claim 1, wherein the source of phosphate ions isselected from the group consisting of phosphoric acid, alkali metalphosphates, and alkaline-earth metal phosphates.
 7. The processaccording to claim 1, wherein the acid is selected from the groupconsisting of hydrochloric acid, sulphuric acid, formic acid, and aceticacid.
 8. The process according to claim 1, wherein the quantity of acidis that necessary to obtain a pH of between 1 and
 2. 9. The processaccording to claim 1, wherein the pH is adjusted with potassiumhydroxide solution, sodium hydroxide solution or mixtures thereof. 10.The process according to claim 1, wherein the pH is adjusted with basesrecovered from industrial operations, without prior purification. 11.The process according to claim 1, wherein the waste waters also containphosphate ions.
 12. The process according to claim 1, wherein the wastewaters are of industrial, agricultural or domestic origin.
 13. Theprocess according to claim 4, wherein the reactant contains between 1.1and 1.5 moles of magnesium and phosphate per mole of ammonia andnitrogen.
 14. The process according to claim 5, wherein the source ofmagnesium ions is magnesium oxide.
 15. A process for treating wastewaters including the addition to waste waters containing more than 1 g/lof ammoniacal nitrogen, and phosphate, of a single liquid reactantcomprising magnesium ions, phosphate ions, and an acid, and thenadjusting the pH of the waste waters to a value of between 9 and 11.